Depression is a difficult mental disorder to treat. Patients having such a disorder are often reluctant to seek the medical attention necessary to diagnose the disorder. Such reluctance is often related to the patient's fear of the stigma associated with seeking psychiatric help or to the patient's feeling of worthlessness associated with depression. Moreover, once the patients seek competent psychiatric help, it is difficult to successfully treat the disorder through psychoanalytic approach alone.
In the Diagnostic and Statistical Manual of Mental disorders, Fourth Edition, (DSM IV) published by the American Psychiatric Association, depressive disorders are classified under mood disorders and are divided into three types: major depressive disorder, dysthymic disorder and depressive disorder not otherwise specified. Major depressive disorder and dysthymic disorder are differentiated based on chronicity, severity and persistence. In major depression, the depressed mood must be present for two weeks. In dysthymic disorder, the depressed mood must be present for two weeks. In dysthymic disorder the depressed mood must be present most days over a period of two years. Usually, major depressive disorder is characterized by its sharp contrast to usual functioning. A person with a major depressive episode can be functioning and feeling normal and suddenly develop severe symptoms of depression. By contrast, a person with dysthymic disorder has chronic depression with less severe symptoms than major depression.
In an effort to treat depression, a variety of antidepressant compositions have been developed. Among these are the selective serotonin reuptake inhibitors (SSRI), such as setraline (registered trademark ZOLOFT.TM.--Pfizer), fluoxetine (registered trademark PROZAC.TM.--Eli Lilly), paroxetine (trade name PAXIL.TM.--Smith Kline Beecham), and fluvoxamine (trade name LUVOX.TM.). Other examples of antidepressant compositions include tricyclic antidepressants such as those sold under the registered trademark ELAVIL.TM. (Merck, Sharpe and Dohme); aminoketone antidepressants such as bupropion; and lithium, a metal used to treat bipolar disorder. However, these drugs are potent, often generating problematic side effects such as lethargy, clouded thinking, a lack of ability to concentrate, and sexual dysfunction. Often, these drugs take about six to eight weeks to exhibit any desirable therapeutic effects. This time period can be prolonged when the correct drug or combinations of drugs has to be determined, by trial and error before any therapeutic effects are observed. Furthermore, current research is beginning to unveil that many of these drugs produce undesirable physiological side effects (Sipgset, O. Drug Saf. 1999. 20(3):277-287; Pache, P. Curr. Med. Chem. 1999. 6(6):469-480), and it is also unknown how these drugs may affect pediatric and adolescent patients (Jensen P. S. Child and Adolescent Research 1999).
Melatonin (N-acetyl-5-methoxytryptamine) is a natural hormone synthesized and secreted primarily by the pineal gland, with highest levels occurring during the dark period of a circadian light-dark cycle. The hormone is also found in the retina and gut. Melatonin is involved in the transduction of photoperiodic information and appears to modulate a variety of neural and endocrine functions in vertebrates, including the regulation of reproduction, body weight and metabolism in photoperiodic mammals, the control of circadian rhythms and the modulation of retinal physiology. The inventors have recently observed the antidepressant-like activity of melatonin in the mouse tail suspension test (Prakhie I. V. and G. F. Oxenkrug 1998).
Pharmacological studies have shown that picomolar concentrations of melatonin selectively inhibit the calcium-dependent release of dopamine from rabbit and chicken retina through the activation of a site having the functional and pharmacological characteristics of a receptor. Using the radioligand, 2-[.sup.125 I]iodomelatonin, melatonin receptor sites have been detected in vertebrate retina (Dubocovich, M. L., Nature, 306:782-784 (1983); Dubocovich, M. L., Eur. J. Pharmacol. 105:193-194 (1984); Dubocovich, M. L., J. Pharmacol. Exp. Ther. 234:395-401, (1985)). Melatonin binding sites have been localized primarily in the suprachiasmatic nucleus and pars tuberalis/median eminence of mammals including humans (Reppert et al., Science 242:78-81 (1988) and Duncan et al., Endocrinol. 125:1011-1018(1989).
While the radioligand, 2-[.sup.125 I]iodomelatonin, is a useful probe for the localization and characterization of melatonin receptors, a significant problem in further elucidating the mechanism of action of melatonin is the lack of potent and selective melatonin receptor agonists and antagonists. Such agonists/antagonists could find application in not only in the study of melatonin receptor interactions but also in the treatment of conditions, possibly affected by melatonin activity, such as depression, jet-lag, disturbances in the sleep-wakefulness cycle, hypertension, glaucoma, reproduction and neuroendocrine disorders.
Generally, agonists of neurotransmitters and neurohormones are structurally related to the transmitter they mimic, whereas antagonists may be structurally unrelated and quite diverse. To date all of the known melatonin agonists are derivatives of melatonin itself, e.g. 2-iodomelatonin, 6-chloromelatonin, 6,7-dichloro-2-methylmelatonin and 8-hydroxymelatonin, all of which contain the 5-methoxy indole ring as an essential moiety. See, Dubocovich, et al., Proc. Nat'l. Acad. Sci. (USA), 84:3916-3918 (1987); Dubocovich, M. L., J. Pharmacol. Exp. Ther., 234:395 (1985); Dubocovich, M. L., Trends Pharmacol. Sci., 16:50-56 (1995).
Membrane associated melatonin sites can be classified into ML.sub.1 and ML.sub.2 subtypes. 5-methoxy-carbonylamino-N-acetyltryptamine (5-MCA-NAT), an indole analogue, has been shown to bind the putative melatonin receptor ML.sub.2 (Dubocovich, M. L., Trends Pharmacol. Sci., 16:50-56 (1995)). This ML.sub.2 -like receptor appears to be linked to the phospholipase-mediated signal transduction pathway through coupling to a G-protein (Popova, J. S. et al., J. Neurochem. 64:130-138 (1995)). The agonist-mediated stimulation of phosphoinositide hydrolysis is antagonized by the melatonin receptor antagonists luzindole (Popova, J. S. et al., J. Neurochem. 64:130-138 (1995)) and N-acetyltryptamine (Eison, A. S., et al. Life Sci. 53:393-398 (1993); Mullins, U. L., et al J. Pineal Res. 17:33-38 (1994)). The physiological function of this putative ML.sub.2 site is difficult to predict at this time, since the localization of 2-[.sup.125 I]iodomelatonin binding sites to specific brain areas has been hampered by technical difficulties.
5-MCA-NAT has been shown to bind with high affinity to ML.sub.2 melatonin sites of hamster brain and shows low affinity or efficacy for ML.sub.1 melatonin receptors (Mollinari, E. J. et al., Soc. Neurosc. Abstr. 20:1168 (1994)). 5-MCA-NAT is classified as a melatonin receptor agonist. The structure of 5-MCA-NAT is: ##STR1##
Therefore, what is needed then, is an effective, pharmacologically-based treatment for depression. It would further be desirable to have a treatment that potentiates the action and reduces the side effects of known compositions used in the treatment of depression. Such a method of treatment is lacking in the prior art.